Shukla Deepak, Ahearn Wendy G, Farid Samir
Research Laboratories, Eastman Kodak Company, Rochester, NY 14650-2109, USA.
J Org Chem. 2005 Aug 19;70(17):6809-19. doi: 10.1021/jo050726j.
Photosensitized electron transfer from a variety of singlet- and triplet-excited donors to N-methoxypyridinium salts leads to N-O bond cleavage. Hydrogen atom abstraction by the resulting methoxy radical from an added alcohol generates an alpha-hydroxy radical that reduces another pyridinium molecule, thus leading to chain propagation. For example, thioxanthone-sensitized reactions of 4-cyano-N-methoxypyridinium, P1, with several aliphatic and benzyl alcohols gave quantum yields for products formation (an aldehyde or a ketone and protonated 4-cyanopyridinium) of approximately 15-20, at reactant concentrations of approximately 0.02-0.04 M. The reaction can also be sensitized with triplet benzopheone, which in this case acts as an electron donor. Energetic limitations on chain propagation are imposed by the relationship between the oxidation potential of the alpha-hydroxy radical and the reduction potential of the pyridinium salt. The chain reactions proceed despite approximately 0.25 eV endothermicity for the electron-transfer step. Chain reactions with the harder-to-reduce 4-phenyl-N-methoxypyridinium, however, are limited in scope because of increased endothermicity for electron transfer. The thioxanthone-sensitized reaction of P1 with benzhydrol was studied in detail by a combination of steady state and transient kinetics. The bimolecular rate constants for the chain propagation reactions:hydrogen atom abstraction by the methoxy radical and electron transfer from the diphenylketyl radical to P1 are approximately 6 x 10(6) and 1.1 x 10(6) M(-1) s(-1), respectively. The kinetic data indicate that deuterium atom abstraction by the methoxy radical from the solvent, acetonitrile-d(3), is a dominant chain-terminating process. Because of a large deuterium isotope effect, approximately 7, the quantum amplification is strongly suppressed when the reaction is carried out in acetonitrile.
从各种单重态和三重态激发供体到N - 甲氧基吡啶鎓盐的光敏电子转移导致N - O键断裂。由此产生的甲氧基自由基从添加的醇中夺取氢原子，生成一个α - 羟基自由基，该自由基还原另一个吡啶鎓分子，从而导致链增长。例如，噻吨酮敏化的4 - 氰基 - N - 甲氧基吡啶鎓（P1）与几种脂肪醇和苄醇的反应，在反应物浓度约为0.02 - 0.04 M时，产物形成（醛或酮以及质子化的4 - 氰基吡啶鎓）的量子产率约为15 - 20。该反应也可用三重态二苯甲酮敏化，在这种情况下二苯甲酮作为电子供体。α - 羟基自由基的氧化电位与吡啶鎓盐的还原电位之间的关系对链增长施加了能量限制。尽管电子转移步骤有大约0.25 eV的吸热性，链反应仍能进行。然而，与较难还原的4 - 苯基 - N - 甲氧基吡啶鎓的链反应，由于电子转移吸热性增加，其范围受到限制。通过稳态和瞬态动力学相结合的方法，对P1与二苯甲醇的噻吨酮敏化反应进行了详细研究。链增长反应的双分子速率常数：甲氧基自由基夺取氢原子以及二苯甲酰基自由基向P1的电子转移分别约为6×10⁶和1.1×10⁶ M⁻¹ s⁻¹。动力学数据表明，甲氧基自由基从溶剂氘代乙腈 - d₃中夺取氘原子是主要的链终止过程。由于大约为7的大氘同位素效应，当反应在乙腈中进行时，量子放大受到强烈抑制。
